The present invention relates to personal computers, and more particularly, relates to expansion boards used to increase the capabilities of such personal computers.
The IBM.RTM. personal computer and its many imitators and competitors have spawned an entire industry geared to the production of equipment to expand the capabilities of personal computers. For example, a typical personal computer, such as those manufactured by IBM.RTM., is manufactured with a system board (sometimes called a motherboard) that includes the digital electronics to provide the basic processing requirements of the computer. The system board typically has a number of female connectors electrically connected to a common bus on the system board so that additional electronics can be provided to communicate with the electronics on the system board. These female connectors are frequently referred to as expansion slots and are used to receive contacts on the edges of printed circuit boards that are plugged into the connectors. In an exemplary personal computer, one expansion slot is used to accommodate a printed circuit board containing digital electronics to control a video display unit, such as a color monitor, or the like. An additional printed circuit board provides a controller for floppy disk drives and/or hard disk drives. Other expansion slots may be used, for example, for printed circuit boards that provide serial interface or parallel interface capability for driving a printer, or the like.
Typically, one end of a printed circuit board that has been plugged into an expansion slot is positioned proximate to the metallic frame of the personal computer. This positioning of the printed circuit card allows the printed circuit card to be structurally connected to the frame of the computer for stability. Furthermore, if the printed circuit card is used to provide input/output capabilities for the computer, an external connector is typically electrically and mechanically connected to the printed circuit card and protrudes through a rectangular opening in the frame of the computer proximate to the end of the card. Thus, an external device, for example, a printer, can be plugged into the external connector to provide electrical interconnection between the external device and the printed circuit card. An exemplary computer typically has a rectangular opening in the frame for each expansion slot.
The metallic frame of the personal computer also provides electromagnetic shielding for the personal computer so that electromagnetic noise generated by rapidly switching signals in the personal computer are not radiated from the computer. The rectangular openings in the frame typically are each approximately 4 inches in height by 1/2 inch in width. Each opening is blocked by a cover that is removed when an expansion card is added to the computer. Since the opening in the frame would permit the emission of radiated electromagnetic energy when a printed circuit card having an external connector is added to an expansion slot, any portion of the opening in the frame not occupied by the connector is covered with a metallic or other electromagnetic shielding material to maintain the integrity of the electromagnetic shielding of the computer.
An exemplary personal computer may have five or more expansion slots. (For example, the IBM Personal Computer XT has eight expansion slots.) In many cases, an expansion slot is required to accommodate the video controller, another expansion slot is required to accommodate the disk controller, another expansion slot accommodates a parallel printer interface, another expansion slot accommodates a serial interface, and another expansion slot accommodates a modem. Other possible combinations of printed circuit boards can of course be accommodated. Irrespective of the combination of expansion boards selected, it is clear that only a limited number of boards can be accommodated at any one time. Although the number of expansion slots has been found to be adequate for many purposes, a number of personal computer users have a need to interconnect a large number of peripheral devices. For example, a user may have a letter quality printer to provide correspondence, a dot-matrix printer to provide drafts of letters and documents, a plotter to provide drawings, and a mouse, or the like, to provide an additional input means as an alternative to the keyboard. Thus, one skilled in the art can readily see that the reasonable needs of some computer users cannot be met with the number of expansion slots available in a typical personal computer.
A typical response to that need has been to combine functions on an expansion card and to provide additional connectors on each expansion card so that more than one peripheral device can be connected to that expansion card. Thus, the number of expansion cards needed can be reduced. For example, expansion cards are available that combine a modem and a serial port, and cards are available that provide a parallel port and a video port, and other combinations. However, the number of combinations that can be provided is limited by the size of the opening in the frame of the personal computer. For example, it can be readily shown that two of the typical 25-pin serial or parallel connectors and their associated mounting hardware cannot be readily accommodated in the opening in the frame.
In order to respond to the need for more input/output connections, a number of products have been developed that utilize the output connectors that will fit through the opening in the frame. An interconnection cable is connected between the output connectors in the expansion card and a separate expansion box that includes additional connectors to accommodate interconnections to additional peripheral devices. In some products, the expansion boxes are passive devices that include a simple switch to selectively interconnect one of the peripheral devices to the internal card at any one time. Thus, the expansion box does not provide the same capabilities as can be provided if each connector is connected directly to a printed circuit board in the computer. Furthermore, the number of connections that can be accommodated are limited by the number of pins in the output connectors on the expansion card. The number of pins is limited by the area of the opening in the frame, and is typically limited to approximately 50 pins, the number of pins in two connectors or one large connector. Other devices have used large cables to extend the entire system bus to an external chassis. This is a very expensive and impractical solution to the problem of increasing the number of input/output connectors.